Descriptionary

banded agate a type of agate characterized by lay­ers of color.

basalt dark, igneous rock formed by volcanoes.

batholith a huge igneous mass—with a surface area of at least 100 square kilometers (40 square miles) and increasing in size as it extends downward—intruded into other rock and found under or within mountain ranges. Also known as a pluton.

bedrock the continuous solid rock exposed at the surface or just beneath the soil or overburden.

benitoite a rare blue gemstone found in California.

beryl a grouping of related gemstones that, in addi­tion to red and green beryl, includes aquamarine, emerald, and morganite.

bitumen a soft coal formed from prehistoric plant material.

black opal a type of quartz that is dark and iridescent.

black pearl a dark pearl ranging in color from gray to peacock green to brown and sometimes artificially treated to enhance color, taken from a mollusk found in the Indo-Pacific Ocean. Also known as Tahitian pearl.

bloodstone a soft type of chalcedony, composed of green jasper with red iron oxide spots. Also known as heliotrope.

blue diamond a rare and valuable diamond con­taining trace amounts of boron, which creates a blu­ish hue. The Hope diamond is the most famous blue diamond.

blueschist metamorphic rock formed under extreme pressure.

blue topaz a brown or colorless topaz until heated, after which it turns blue.

Bohemian diamond a fake diamond, actually rock crystal.

Bohemian ruby a fake ruby, actually a pyrope garnet.

bort industrial grade diamonds.

boulder train a line of boulders following the his­torical path of a glacier.

breccia broken pieces of rock cemented or bonded together with other types of rock.

cairngorm a yellowish-brown smoky quartz, once used in the manufacture of Celtic jewelry.

calcite a crystalline form of calcium carbonate.

calcium carbonate derived from deposits of sea- shells, the major component of limestone and chalk.

California ruby a fake ruby, actually a pyrope garnet.

canary diamond a variety of diamond character­ized by its deep yellow color.

carat unit of weight used in weighing precious gems; 1 carat equals 0.2 gram. Also the measure of the purity of gold; 24 carats equals pure, 100 percent gold; 18 carat gold is 75 percent gold and 25 percent alloy; and so forth.

carbonaceous of sedimentary rock, containing dark organic matter.

carbonado an opaque black diamond used in drill bits. Carbonados, which are mined in South America and Africa, are thought to have been deposited by an asteroid impact 3 billion years ago.

carnelian a variety of chalcedony, characterized by its translucent red or orange color, which may be banded with either color.

cat's eye otherwise known as chatoyant chrysoberyl, a yellow or greenish-yellow stone, characterized in its center by what resembles a slitlike pupil of a cat.

chalcedony colored quartz with a milky appearance.

chalk derived largely from fossil seashells, a soft, light-colored calcite.

champagne diamond a pinkish-brown diamond.

chatoyancy in a gemstone, having a cat's eye-like effect, specifically from inner bands of reflected light.

chatoyant chrysoberyl see cat's eye.

chemical weathering the erosion of rocks through oxidation and hydration.

chert fine-grained quartz, once used by Native Americans for spearheads, arrowheads, and knives.

chrysoberyl a yellow-green crystal mineral.

chrysolite a transparent variety of olivine, some­times sold as gemstones and thought by some to have magical qualities.

cinnabar the mineral mercury sulfide, characterized by color ranging from cinnamon to brick red.

cinnamon stone a brownish variety of garnet. Also called essonite.

citrine a type of quartz characterized by its yellow or orange color, sometimes mistaken for imperial topaz.

coal sedimentary rock formed by deposits of com­pressed plant matter.

cognac diamond a brownish-colored diamond.

conglomerate a sedimentary rock composed of a cluster of particles, gravel, and pebbles held together by clay.

corundum an extremely hard family of stones of crystallized aluminum that includes rubies and sapphires.

country rock rock into which magma has been intruded.

crystallize to turn to crystal or to form crystals.

Culinan diamond a world-record diamond, weigh­ing 3,106 carats, found in South Africa in 1905. Also known as the Star of Africa.

Cupid's darts see rutilated quartz.

dacite light-colored, igneous volcanic rock.